Hydrothermal alteration and tectonic evolution of an intermediate- to fast-spreading back-arc oceanic crust: Late Ordovician Solund-Stavfjord ophiolite, western Norway

Autor: Harald Furnes, Karlis Muehlenbachs, Yildirim Dilek, Hege Fonneland-Jorgensen
Rok vydání: 2005
Předmět:
Zdroj: The Island Arc. 14:517-541
ISSN: 1440-1738
1038-4871
DOI: 10.1111/j.1440-1738.2005.00481.x
Popis: The Solund-Stavfjord ophiolite complex (SSOC) in western Norway represents a remnant of the Late Ordovician oceanic lithosphere, which developed in an intermediate- to fast-spreading Caledonian back-arc basin. The internal architecture and magmatic features of its crustal component suggest that the SSOC has a complex, multistage sea floor spreading history in a supra-subduction zone environment. The youngest crustal section associated with the propagating rift tectonics consists of a relatively complete ophiolite pseudostratigraphy, including basaltic volcanic rocks, a transition zone between the sheeted dyke complex and the extrusive sequence, sheeted dykes, and high-level isotropic gabbros. Large-scale variations in major and trace element distributions indicate significant remobilization far beyond that which would result from magmatic processes, as a result of the hydrothermal alteration of crustal rocks. Whereas K 2O is strongly enriched in volcanic rocks of the extrusive sequence, Cu and Zn show the largest enrichment in the dyke complex near the dyke-volcanic transition zone or within this transition zone. The δ 18 O values of the whole-rock samples show a general depletion structurally downwards in the ophiolite, with the largest and smallest variations observed in volcanic rocks and the transition zone, respectively. δ 18 O values of epidote-quartz mineral pairs indicate 260- 290°C for volcanic rocks, 420 °C for the transition zone, 280-345 °C for the sheeted dyke complex and 290-475°C for the gabbros. The 87 Sr/ 86 Sr isotope ratios show the widest range and highest values in the extrusive rocks (0.70316-0.70495), and generally the lowest values and the narrowest range in the sheeted dyke complex (0.70338-0.70377). The minimum water/rock ratios calculated show the largest variations in volcanic rocks and gabbros (approximately 0-14), and generally the lowest values and range in the sheeted dyke complex (approximately 1-3). The δD values of epidote ( -1 to -12‰), together with the δ 18 O calculated for Ordovician seawater, are similar to those of present-day seawater. Volcanic rocks experienced both cold and warm water circulation, resulting in the observed K2O-enrichment and the largest scatter in the δ 18 O values. As a result of metal leaching in the hot reaction zone above a magma chamber, Zn is strongly depleted in the gabbros but enriched in the sheeted dyke complex because of precipitation from upwelling of discharged hydrothermal fluids. The present study demonstrates that the near intact effect of ocean floor hydrothermal activity is preserved in the upper part of the SSOC crust, despite the influence of regional lower greenschist facies metamorphism.
Databáze: OpenAIRE
Externí odkaz: